This is a known issue currently affecting ESXi 6.7 release and currently there is no resolution.
Workaround:
Enable IPFIX firewall rule on ESXi host from Web Client UI
1. Login to UI of ESXi
2. Navigate to networking tab on the left pane
3. Navigate to Firewall rules tab on the right pane
4. Identify the ipfix firewall rule from the displayed rules and go to action and enable the same.